I had chlorine gas exposure once as a result of cleaning a surface that had residual of another cleaner on it. When you use chlorine, you’re not supposed to mix certain other chemicals w/it that liberate the gas (ammonia does that.) I’m thinking error or ill-informed prankster. You’re right, though. Intentional harm didn’t even occur to me.
Most larger pools these days use a salt-based chlorinator. However, I suppose this one could still be using the older powdered chemicals I used when I was a lifeguard. It’s large-scale chemistry, and all it takes is an error in the schedule or a malfunction in the automatic system to end up with a huge shock of chlorine in the water.
Chlorax and uric acid will do the same. It is the same thing as mixing chlorine and ammonia.